Tuesday, August 30, 2011

For 20 years, astronomers have been trying to come up with a simulated galaxy that comes close to the look of the Milky Way and other spiral galaxies, but fell short of the mark. Now a team of astrophysicists from the University of Zurich together with astronomers from the University of California at Santa Cruz, has produced the world’s first realistic simulation of the formation of our home galaxy, the Milky Way. The research group is jointly run by Lucio Mayer, an astrophysicist at the University of Zurich, and Piero Madau, an astronomer at University of California at Santa Cruz.

Previous efforts to form a massive disk galaxy like the Milky Way had failed, because the simulated galaxies ended up with huge central bulges compared to the size of the disk. Mayer and his colleagues were more successful in part because of the computer firepower at their disposal: the high-performance supercomputers Cray XT5 “Monte Rosa” at ETH Zurich’s Swiss National Supercomputing Center (CSCS) and the NASA’s Pleiades supercomputer clocking a total of 1.4 million processor-hours. On a regular PC the calculations would have needed 570 years to complete.

For the simulation, the team developed a high-resolution model that simulates the formation of a galaxy with 790 billion solar masses from which gas, dark matter and stars form. The effort used a software platform known as Gasoline to trace the motions of more than 60 million particles, representing galactic gas as well as dark matter, over the course of more than 13 billion years.

Even though the simulation is of very high resolution, by human standards, its 60 million particles is a pale comparison against the hundreds of billions of stars in the Milky Way. The universe is amazing.

For their study, the scientists developed a highly complex simulation in which a spiral galaxy similar to the Milky Way develops by itself without further intervention. Named after Eris, the Greek goddess of strife and discord, because of the decades of debate surrounding the formation of spiral galaxies, the simulation offers a glimpse in time lapse into almost the entire genesis of a spiral galaxy. Its origins date back to less than a million years after the Big Bang. “Our result shows that a realistic spiral galaxy can be formed based on the basic principles of the cold dark matter paradigm and the physical laws of gravity, fluid dynamics and radiation physics,” explains Mayer.

On the left the simulated galaxy after 13 billion of years. The gas clouds are represented in red, the stars in blue. On the right as comparison the spiral galaxy M74 that is similar to the Milky Way (University of Zurich and NASA)

The simulation also shows that in an entity that is supposed to develop into a spiral galaxy, the stars form in the areas where giant cloud complexes are. In these cold giant molecular clouds, the gas exhibits extremely high densities. The star formation and distribution there does not occur uniformly, but rather in clumps and clusters. This in turn results in a considerably greater build-up of heat through local supernova explosions. Through this massive build-up of heat, visible standard matter is removed at high redshift. This prevents the formation of a bulge in the center of the galaxy. The removal of baryonic matter, as the visible standard matter is also known, also reduces the overall mass of the gas present at the center. This results in the formation of the correct stellar mass, as can be observed in the Milky Way. At the end of the simulation, a thin, curved disk results that corresponds fully to the astronomical observations of the Milky Way in terms of the mass, angular momentum and rotation velocity ratios.

The new simulation confirms the results for the formation of disk-shaped dwarf galaxies published by Mayer and demonstrates that the model – unlike all previous approaches – can recreate both small and extremely large galaxies realistically. Moreover, from the simulation it an also be deduced that protogalaxies with a large disk made of gas and stars at the center already formed a billion years after the Big Bang, and therefore long before our present galaxies.

The simulation also predicts stars and gas for the outer halo of the Milky Way six hundred thousand light years away. Only the next generation of space probes and telescopes will be able to detect these extremely faint stars. Furthermore, the simulation makes predictions with regard to the radial distribution of hot gas around the galaxy´s central disk. Future telescopes that can measure X-rays, as the IXO Mission of the European Space Agency (ESA) is planning, for example, will test these predictions.